As is known in the art, a radio frequency (RF) amplifying device (such as a power amplifier used in RF systems, for example) has a first amplifying region in which signals are linearly amplified (often referred to as the amplifier's “linear region”) and a second amplifying region in which the amplifying devices exhibit non-linear characteristics (often referred to as the amplifier's “non-linear region” or “saturation region”).
When operating in its non-linear region, the amplifying device causes distortion in the phase and amplitude of an output signal. For example, an amplifier operating in its non-linear region may generate inter-modulation products. Such distortion is not desirable in most applications as it can lead to a degradation in performance of a system which includes the amplifier.
Such distortion components may be reduced or even eliminated by operating the amplifier in its linear region. One problem with the approach of operating the amplifier in its linear region, is that the amplifier may have only a limited range of RF input signal power levels over which it provides linear amplification. Furthermore, amplifiers are much more efficient (e.g. in terms of power added efficiency) when they are operating at or near their non-linear region but less efficient when operating in their linear region. Thus, the approach of operating the amplifier in its linear region can be quite limiting and not appropriate for many RF applications.
Power amplifiers, for example, often operate near their saturation region where amplifiers work at the maximum efficiency and thus may exhibit strong non-linear characteristics. Thus, in order to maximize output power and efficiency, the gains and phases of power amplifier output signals are distorted.
Consequently, power amplifiers often utilize a linearization circuit (or more simply a “linearizer”) for compensating non-linear characteristics of a power amplifier. So-called “feed-forward” linearizers and “pre-distortion” linearizers have been conventionally proposed.
In the case of a power amplifier using a feed-forward linearizer, signals are dividedly applied to a main path and a sub-path, and carrier signals (or a tone signal and its corresponding signals) on the main path are amplified to a predetermined level by a main amplifier as the power amplifier and then output.
Intermodulation signals of the main amplifier are selectively output by a 3 dB hybrid coupler and attenuated to a predetermined level by an attenuator. The 3 dB hybrid coupler offsets the attenuated signals and signals that are applied to the sub-path and delayed via a first delay loop, so that the intermodulation signals are synthesized.
The resulting signals that are synthesized by the 3 dB hybrid coupler are applied to an error amplifier so that errors of the synthesized signals are corrected and the corrected signals are amplified. Thereafter, the corrected and amplified signals are amplified on the main path and synthesized with signals, which are delayed by a predetermined time via a second delay loop, and output. In the synthesization process, intermodulation distortion (MD) signals are offset and output.
Meanwhile, in the case of a power amplifier using a pre-distortion linearizer, an applied carrier signal is pre-distorted beforehand by a predetermined pre-distorter. The pre-distorted signal is amplified to a predetermined level by a main amplifier and output. In other words, a pre-distorted signal is generated beforehand and offset by a pre-distorted signal portion of an applied signal, and the remaining portion of the applied signal is amplified and output. In general, the power amplifier using the pre-distortion linearizer can have a small and lightweight structure with a broad bandwidth and a wide operating range at low cost.
While the conventional techniques described above have been somewhat effective, they have utilized relatively complicated circuits and techniques and are relatively expensive to implement both in terms of dollar cost and manpower cost.